Conventionally, there are known, as the hybrid construction machine, a construction machine employing a series hybrid system, and a construction machine employing a parallel hybrid system.
An example of a construction machine employing a series hybrid system is disclosed in patent literature 1. Specifically, the construction machine disclosed in patent literature 1 is provided with an engine, a generator to be driven by the engine, a motor to which electric power generated by the generator is supplied, and a hydraulic pump to be driven by the motor. In the construction machine employing a series hybrid system, the engine and the hydraulic pump are not mechanically connected to each other.
On the other hand, the construction machine employing a parallel hybrid system is provided with an engine, a hydraulic pump to be driven by the engine, and a generator-motor which is operated as a generator by the power of the engine for storing electric power in a battery, and which is operated as a motor by the electric power from the battery for assisting the engine which drives the hydraulic pump.
In the construction machine employing a series hybrid system, however, the engine and the hydraulic pump are not mechanically connected to each other. Therefore, it is impossible to directly utilize the power of the engine as the power of the hydraulic pump. Accordingly, high output is required for the motor in order to drive the hydraulic pump, which results in an increase in the size of the motor. As a result, the degree of freedom of layout may be lowered. Further, even if there is sufficient output of the engine, it is impossible to directly transmit the engine power to the hydraulic pump. Further, when disorder occurs in the electrical system, it is difficult to perform emergency evacuation by driving the hydraulic pump.
On the other hand, in the construction machine employing a parallel hybrid system, one generator-motor serves as a generator and a motor. Therefore, it is impossible to efficiently use the generator-motor. Specifically, whereas the output of a generator-motor at the time of generating electric power, and the output of the generator-motor at the time of assisting the engine differ from each other, one rated output is set for the generator-motor. Accordingly, it is impossible to effectively utilize the rated output at the time of generating electric power and at the time of assisting the engine.